|The human need for sleep is a mystery.|
But no one knows what sleep actually does.
"It's sort of embarrassing," said Dr. Michael Halassa, a neuroscientist at New York University. "It's obvious why we need to eat, for example, and reproduce … but it's not clear why we need to sleep at all." [5 Surprising Sleep Discoveries]
We're vulnerable when we're asleep, so whatever sleep does, it must be worth the risk of the brain taking itself mostly offline. There are a few theories about why we sleep, and although none of them are totally solid, a few try to explain what happens each night, pulling in research on topics ranging from cellular processes to cognition. Researchers say it does seem clear that sleep is key to the brain's ability to reorganize itself — a feature called plasticity.
It's not hard to prove that sleep is important. Rats totally deprived of sleep die within two or three weeks, according to research by the pioneering University of Chicago sleep scientist Allan Rechtschaffen. No one has done similar experiments on humans, for obvious reasons, but a 2014 study published in The Journal of Neuroscience found that a mere 24 hours of sleep deprivation caused healthy people to have hallucinations and other schizophrenia-like symptoms.
One reason it is difficult to get a handle on why we sleep is that sleep is actually pretty difficult to isolate and study. Sleep-deprivation studies are the most common way to study sleep, said Marcos Frank, a neuroscientist at the University of Washington, but depriving an animal of sleep disrupts many of its biological systems. It's hard to tell which outcomes are directly attributable to sleep deprivation rather than, say, stress.
Another reason sleep is hard to understand is that the brain may be doing two different things during the two major stages of sleep. As the night wears on, sleepers cycle through non-rapid eye movement (non-REM) and rapid-eye-movement (REM) sleep. Non-REM sleep is marked by slow brain waves called theta and delta waves. In contrast, the brain's electrical activity during REM sleep looks much like it does when a person is awake, but the muscles of the body are paralyzed. (If you've ever experienced sleep paralysis, it's because you woke from REM sleep before this paralysis ended.)
Studies have found differences in the biology of the brain during these different stages. For example, during non-REM sleep, the body releases growth hormone, according to a 2006 review of the biology of sleep published by Frank in the journal Reviews in the Neurosciences. Also during non-REM sleep, the synthesis of some brain proteins increases, and some genes involved in protein synthesis become more active, the review found. During REM sleep, in contrast, there does not appear to be any increase in this sort of protein-producing activity.
What do we know about sleep?
One conclusion that has emerged from sleep research is that sleep does appear to be largely a brain-focused phenomenon, Frank said. Although sleep deprivation affects the immune system and alters hormone levels in the body, its most consistent impacts across animals are in the brain. [10 Things You Didn't Know About the Brain]
"The central nervous system is always impacted by sleep," Frank said. "There may have been other things that evolution added onto the primary function of sleep, but the primary function of sleep probably has something to do with the brain."
There is some evidence, in fact, that sleep is just something that neurons do when they're joined in a network. Even neuron networks grown in lab dishes show stages of activity and inactivity that sort of resemble waking and sleeping, Frank said. That could mean sleep arises naturally when single neurons begin to work together.
This could explain why even the simplest organisms show sleep-like behaviors. Even Caenorhabditis elegans, a tiny worm with only 302 neurons, cycles through quiet, lethargic periods that look like sleep. Perhaps the first simple nervous systems to evolve exhibited these quiet periods, Frank said, and as brains got larger and more complex, the state of inactivity also had to get more complicated.
"It would be very disadvantageous to have a complex brain like ours where different parts are falling in and out of sleep, so you need to have some way to orchestrate this," he said.
What happens during sleep?
But the idea that sleep is a natural property of neuron networks doesn't really explain what's going on during sleep. On that front, scientists have a number of theories. One is that sleep restores the brain's energy, according to a 2016 review in the journal Sleep Medicine Reviews. During non-REM sleep, the brain consumes only about half the glucose as it does when a person is awake. (Glucose is the sugar that cells burn up to release energy.)
But if the idea that sleep restores brain energy is true, the relationship between sleep and the brain's energy usage is not straightforward. For example, during sleep deprivation, the brain's breakdown of an energy source called glycogen increases in some parts of the brain but decreases in others. More research is needed to understand this link. [The 7 Biggest Mysteries of the Human Body]
Another idea is that sleep might enable the brain to clear out toxic products produced when we're awake. The brain is a huge consumer of energy, which means it also produces much waste. Some recent research suggests that sleep is a time when the brain sweeps itself clean, Frank said, but those results need to be replicated.
"It might be something that kind of happens with sleep," Frank said, "but it may not be the most important thing sleep is doing."
Perhaps the most promising theory of sleep so far is that it plays a major role in the brain's connectivity and plasticity. Plasticity is involved in learning and memory. Although it's unclear exactly how, plenty of evidence suggests that losing sleep can cause problems with memory, particularly working memory, the process that allows people to hold information in an easily accessible way while working out a problem. People who are sleep-deprived also struggle with choosing what to pay attention to and regulating their emotions.
One way sleep may affect the brain's plasticity is through its effects on the synapses, or connections between neurons. Research has shown that when animals learn a new task, their neurons seem to strengthen the synaptic connections involved in learning that task during the next sleep cycle, according to the Sleep Medicine Reviews paper. In experiments where researchers put a patch over one of an animal's eyes, the brain circuits associated with visual information from that eye weakened within hours, according to research by the University of Surrey's Julie Seibt and colleagues. REM sleep, however, strengthened the circuits involving the other eye, suggesting that the brain uses sleep to adjust to changing inputs. [7 Weird Facts About Balance]
"It could still mean there is something really basic and central at the heart of [sleep], something basic that brain cells have to do, and one outcome is the plastic change," Frank said.
In the future, a better understanding of sleep may come from research on cells called glia cells, Frank said. These brain cells, whose name literally means "glue," were once thought to be largely inert but have been recently discovered to have a range of functions. Glia cells outnumber neurons by up to three to one, Frank said. Glia cells may control the flow of cerebrospinal fluid throughout the brain, which could result in clearing out metabolic waste during sleep, for example.
"It could be that the mystery of sleep could be solved by understanding what these very specialized glia cells are doing," Frank said.